Generally, a servocontrol arrangement comprises electronic components which are liable, in failed mode, to bring about a spurious signal which may make the servocontrolled control surface oscillate. A phenomenon of this type is called an “oscillatory fault”. Another possible cause of the oscillation is the malfunction or the breakage of a mechanical part of the actuator.
It is known moreover that when such an oscillatory fault exhibits a frequency which lies inside the passband of the actuator, its effect may be to bring about significant loads on the structure of the aircraft, thus necessitating a strengthening of this structure, if a dedicated device does not make it possible to detect this fault. Further, if an oscillatory fault exhibits a frequency which lies inside the passband of the actuator, its effect may be to bring about excessive loads in the case of excitation of one of the natural modes of vibration of the aircraft (phenomenon of resonance, aeroelastic coupling), and this may bring about, in the extreme case, damage to the structure of the aircraft. In addition, if an oscillatory fault exhibits a frequency which lies inside the passband of the actuator, its effect may be to accelerate the fatigue of the actuator or actuators used, and to reduce the comfort of the aircraft's passengers.
The complete coverage of such oscillatory faults would require overly expensive strengthening of structures of the aircraft. In practice, the aircraft is designed to absorb oscillatory faults of a certain amplitude, as a function of frequency. Hence, monitoring may be put in place to guarantee that the vibrations of the aircraft remain inside a predetermined amplitude/frequency envelope.
However, the routine solutions for carrying out such monitoring exhibit high dependency with respect to the hardware used; to the type of law for piloting the aircraft (function of the flexibility or otherwise of the latter); to the computer's acquisition and generating system; and to said computer's fault modes.
Consequently, to a particular family of aircraft there corresponds on each occasion a particular routine solution, which does not exhibit any guarantee of being applicable to another, existing or future, family of aircraft.
Moreover, the routine monitoring solutions generally exhibit restricted coverage, usually only carrying out detection of the oscillations brought about by a particular component of the servocontrol arrangement.
French Patent No. FR-2 893 911 makes it possible to remedy these drawbacks at least partially. It relates to a method, which is robust and which is applicable to any type of aircraft with electric flight controls, for detecting an oscillatory fault in a positional servocontrol arrangement of a control surface of the aircraft, in particular of a transport airplane, said method making it possible to detect an oscillatory fault of a minimum amplitude in a number of limited periods, and to do so whatever the frequency of this oscillatory fault.
According to French Patent No. FR-2 893 911, for this purpose, the following series of successive steps is carried out in an automatic and repetitive manner:    a) a theoretical position corresponding to a reference position of said control surface in the absence of any fault is estimated with the aid of a control surface control order which feeds a model of the actuator;    b) the difference between said theoretical position estimated in step a) and the actual position measured by the sensor is computed so as to form a residual value; and    c) this residual value is compared with at least one predetermined threshold value, a counting is carried out of all the successive and alternate oversteppings of said predetermined threshold value by said residual value, and, as soon as the number resulting from said counting becomes greater than a predetermined number, an oscillatory fault is detected which represents a periodic signal of sinusoidal type, whose frequency, amplitude and phase follow a uniform law (that is to say do not exhibit any favored values).
French Patent No. FR-2 893 911 therefore envisages comparing the real operation of the monitored servocontrol arrangement (which is illustrated by the measured actual position), with fault-free expected ideal operation (which is illustrated by said theoretical position), thereby making it possible to highlight any oscillatory fault when it arises. This comparison is performed by computing a residual value. Consequently, by virtue of the method of detection of French Patent No. FR-2 893 911, it is possible to detect, in the monitored servocontrol arrangement, any oscillatory fault of a given minimum amplitude in a given number of periods.
Moreover, Patent Application No. FR-08 05131 discloses a method of detection based on that of French Patent No. FR-2 893 911 and envisaging reinjecting into the actuator model the residual value, multiplied by a particular gain, so as to accelerate the convergence of the estimation and to produce a residual value of lower amplitude.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.